Communication networks often require bidirectional transmission links. The most direct implementation in fiber optics is to use two unidirectional waveguides, one waveguide for data transmission in a first direction, the other waveguide for data transmission in the reverse direction, with distinct transmit and receive devices on either end of each of the waveguides. Alternatively, a single waveguide can be used with optical splitter at opposite ends of the waveguide to provide optical paths to transmitters and receivers at each end of the waveguide. Optical splitter can be designed to have directional coupling characteristics, i.e. signals to the far end ("go" signals) can be separated from signals from the far end ("return" signals), enabling full-duplex communications. However, reflection points in the link, such as connectors, lead to cross talk between the communications paths. The suppression of crosstalk requires extra effort, such as the application of wavelength division multiplexing (WDM) or frequency division multiplexing (FDM). The former utilizes different optical carriers, the latter utilizes different electrical carriers for the go and return paths.
As an alternative, it has been suggested that a laser diode, or light emitting diode (LED) could be used as both a light transmitter and a light receiver. This would thereby remove the need for optical splitter and photodiodes at the ends of the optical waveguide to separate transmit and receive paths. The potential benefits of using a single device as both an emitter and a detector are great in that the parts count and assembly time are reduced considerably. However, transmission systems to date, using such a concept, have not been capable of full-duplex transmission.
An example of a method and apparatus that has been created for the purpose of bidirectional communication over a single waveguide can be found in U.S. Pat. No. 4,879,763 issued Nov. 7, 1989 in the name of T. H. Wood, entitled "Optical Fiber Bidirectional Transmission System", in which a bidirectional optical communications system is described using a multiple quantum well structure as both a photodetector and light modulator.
Technologies for using a laser or light emitting diode (LED) as a light emitter and detector are described in the following U.S. patents: U.S. Pat. No. 4,773,074 issued Sep. 20, 1988 in the name of Hunsperger et al., entitled "Dual Mode Laser/Detector Diode for Optical Fiber Transmission Lines", in which a semiconductor diode device for direct optical coupling to an optical signal transmission apparatus is disclosed; U.S. Pat. No. 4,195,269 issued Mar. 25, 1980 in the name of Ettenberg et al., entitled "Two-way Single Fiber Optical Communication System", in which is disclosed an injection laser whose characteristics vary upon radiation impinging on the laser such that the laser operates as a light detector; and U.S. Pat. No. 4,687,957 issued Aug. 18, 1987 in the name of V. P. O'Neil, entitled "Fiber Optic Transceiver", in which a half-duplex transceiver circuit is disclosed in which a single diode acts as both the light emitter and the light detector.
Since an optical device can only act as an emitter or detector at any one point in time, but not simultaneously, half-duplex seems inevitable for bidirectional transmission over a single fiber. The present invention proves otherwise.